Impersonal Pronoun "One"

We can use "one" to talk about people in general or for unspecified objects.

One can’t learn a language in six weeks.
If one (somebody, you) wishes to make oneself popular, one must work very hard for it.
One is knocking at the door.(not general, it is specified)
Somebody is knocking at the door.

Also use "They" or "You" in the same way

They speak English in this part of the country.
They don't respect teachers in some schools.

You really can't find anything in this shop. (I may be talking to myself)
One really can't find anything in this shop.

If one gets ill, one should see a doctor. (Sb. he / she / they)

One is knocking at the door.
Somebody (not in general, it is a specified)
The newspapers that most men like are the ones with most sports pages.

If one gets sick, one should see a doctor.

Attention:

If you begin your sentence with "one", you should continue using "one, one's, oneself..." when referring to the same person.

If one wants to be happy, one should not be greedy.
If one wants to succeed, one should study hard.

In the sentence below however, we can substitute "he", "he or she" or "they".

If someone wants to succeed, he should study hard.If someone wants to succeed, he or she should study hard.If someone wants to succeed, they should study hard.

Impersonal Pronoun It

It is one of the most commonly used impersonal pronouns in English. It can refer to many unspecified subjects depending on the context.

It is very cold today. (the weather)
It is Saturday today.
It is 5 o’clock somewhere.
It is always snowy in Lake Tahoe.

Infinitive Subject

It is impossible to beat them. (To beat them is impossible)
It is unbelievable!

To emphasize

It was George who saved your life!
It was me who wrote the essay.(I did it!)